Fish physiology

A few other fish have structures resembling labyrinth organs in form and function, most notably snakeheads, pikeheads, and the Clariidae catfish family.

Fish dependent solely on dissolved oxygen, such as perch and cichlids, quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud.

[11] Fish gill slits may be the evolutionary ancestors of the tonsils, thymus gland, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches.

The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids.

In slow-moving or bottom dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth.

The ventricle is another thick-walled, muscular chamber and it pumps the blood, first to the fourth part, bulbus arteriosus, a large tube, and then out of the heart.

It then moves to the intestine, where the process of breaking the food down into simple molecules continues and the results are absorbed as nutrients into the circulatory and lymphatic systems.

It commonly has a number of pyloric caeca, small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food.

In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns.

[22] In fish, there is no true large intestine, but simply a short rectum connecting the end of the digestive part of the gut to the cloaca.

The valve provides a long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the cloaca.

Estrogenic compounds found in pesticides, birth control, plastics, plants, fungi, bacteria, and synthetic drugs leeched into rivers are affecting the endocrine systems of native species.

[26] In Boulder, Colorado, white sucker fish found downstream of a municipal waste water treatment plant exhibit impaired or abnormal sexual development.

[29][30][31] In the marine ecosystem, organochlorine contaminants like pesticides, herbicides (DDT), and chlordan are accumulating within fish tissue and disrupting their endocrine system.

A marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt.

Salmon has been observed to inhabit two utterly disparate environments — marine and fresh water — and it is inherent to adapt to both by bringing in behavioral and physiological modifications.

In contrast to bony fish, with the exception of the coelacanth,[34] the blood and other tissue of sharks and Chondrichthyes is generally isotonic to their marine environments because of the high concentration of urea and trimethylamine N-oxide (TMAO), allowing them to be in osmotic balance with the seawater.

The common thresher shark has a similar mechanism for maintaining an elevated body temperature, which is thought to have evolved independently[failed verification].

This allows the tuna to elevate the temperatures of the highly aerobic tissues of the skeletal muscles, eyes and brain,[40][41] which supports faster swimming speeds and reduced energy expenditure, and which enables them to survive in cooler waters over a wider range of ocean environments than those of other fish.

The red myotomal muscles derive their colour from myoglobin, an oxygen-binding molecule, which tuna express in quantities far higher than most other fish.

A typical characteristic of many animals that utilize undulatory locomotion is that they have segmented muscles, or blocks of myomeres, running from their head to tails which are separated by connective tissue called myosepta.

A higher initial angle of orientation and more dorsoventral bulging produces a faster muscle contraction but results in a lower amount of force production.

This phenomenon results in an architectural gear ratio, determined as longitudinal strain divided by fiber strain (εx / εf), greater than one and longitudinal velocity amplification; furthermore, this emergent velocity amplification may be augmented by variable architectural gearing via mesolateral and dorsoventral shape changes, a pattern seen in pennate muscle contractions.

Sharks have keen olfactory senses, located in the short duct (which is not fused, unlike bony fish) between the anterior and posterior nasal openings, with some species able to detect as little as one part per million of blood in seawater.

[66] In 2003, Scottish scientists at the University of Edinburgh and the Roslin Institute concluded that rainbow trout exhibit behaviors often associated with pain in other animals.

[72] Oogonia development in teleost fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilisation processes.

[73] Postovulatory follicles are structures formed after oocyte release; they do not have endocrine function, present a wide irregular lumen, and are rapidly reabsorbed in a process involving the apoptosis of follicular cells.

In the majority of these species, fertilisation takes place outside the mother's body, with the male and female fish shedding their gametes into the surrounding water.

The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed metamorphosis) to become juveniles.

[79] A past laboratory experiment done on cyprinids has established that the time budget for social behaviour within a shoal varies proportionally to the quantity of fishes present.